The long term goal of the proposed research is to define the molecular mechanisms involved in the transition from latent to lytic herpes simplex virus (HSV) gene transcription. They previously developed the hyperthermic stress (HS) reactivation model which is unique in that the production of infectious virus is detectable within 12-14 hours after the induction stimulus. Using this model, they have identified what is likely to be a key event in the regulation of reactivation, namely the up regulation of ICPO within 1 hour post HS. Insight into the molecular regulation of reactivation must ultimately be obtained from analysis of individual latently infected neurons. They have developed a new method, contextual expression analysis, CXA, to obtain quantitative information about the DNA and RNA in individual cells within solid tissues. In this proposal, the power of PCR and RT PCR will be harnessed through CXA to construct a molecular definition of latency and reactivation. Their ability to precisely quantify the number of latently infected neurons in the ganglia and examine the RNA and DNA content will allow them to meaningfully evaluate wild type and genetically engineered mutant strains to achieve the following specific aims: (1) Determine the impact of the number of latently infected neurons and/or the number of viral genome copies within individual latently infected neurons and/or the number of viral genome copies within individual latently infected neurons upon the initiation and progression of HS inducted reactivation in vivo; (2) Utilize CXA-RNA strategies to characterize viral transcription during latency and following HS induced reaction at the neuronal population and single cell level; (3) Determine the biological significance and biochemical basis of the rapid up regulation of the ICPO gene following HS induced reaction in vivo. Defining the regulatory mechanisms by which the "latent" repository of viral genetic information periodically give rise to infectious virus is central to understanding this important aspect of the viral life cycle. Insight into these viral functions could contribute significantly toward our ability to design effective vaccines, develop treatments for the prevention of recurrent disease, and efficiently transfer, maintain and regulate foreign genes in the human host.